Use of a process fluid with an environmentally compatible biostabilizer in a geothermal borehole

ABSTRACT

The present invention relates to the use of a process fluid with an environmentally compatible biostabilizer in a geothermal borehole. The biostabilizer is characterized in that it comprises at least one organic acid, or a salt, alcohol or aldehyde thereof, wherein the at least one organic acid is selected from the group consisting of hop acids, resin acids, fatty acids and mixtures thereof. The biostabilizer is preferably a mixture of hop extract, rosin and myristic acid. The invention further relates to related process fluids and methods for producing the same.

The invention relates to the use of a process fluid in a geothermalborehole, wherein said process fluid includes a biostabilisater.

Geothermics means the technical utilisation of geothermal energy.Geothermal energy is the heat stored in the earth's crust and ranksamong the renewable energy sources. It may be utilised either directly(e.g. by means of a heat pump) or indirectly (e.g. for generatingelectric current). A distinction is drawn between near-surfacegeothermics (with drills to a depth of up to 400 metres) and deepgeothermics (with drilling depths from 400 metres on and usually down to4000 metres or 5000 metres). A bore to exploit geothermal deposits iscalled a geothermal bore. In low-enthalpy geothermal heat deposits, asare available for example in the Alpine area, deep geothermal bores arerequired as a rule. For that matter, the geothermal drilling method (toproduce a geothermal borehole) is within the scope of the termgeothermics, as used herein, or, when the drilling depth is deeper than400 metres, is within the scope of deep geothermics, as used herein. Ingeothermics, process fluids (i.e. fluids which are used in a process ormethod without getting consumed normally) are made use of which usuallyinclude water, for example as a drilling fluid in exploiting thegeothermal deposit or as a heat carrier during operating (i.e.exploiting or utilising) the geothermal deposit.

In geothermics, the Hot Dry Rock technique (also known as “EnhancedGeothermal System” or “Hot Fractured Rock”) plays a distinctive role. Inthis process, a process fluid (usually on an aqueous base) is circulatedbetween at least two adjacent geothermal boreholes. During theoperation, a colder process fluid is fed to the geothermal deposit (i.e.the geological formation) via the first geothermal borehole and iswithdrawn again at the second geothermal borehole in a warmer state,possibly enriched by naturally present deep water. By pressing in theprocess fluid under high pressure (usually up to 150 bar), the fissurespresent in the geological formation get widened, and new ones are formedin some circumstances, which massively increases the surface availablefor an heat exchange between the fluid and the formation. Different towhat is common in for example Hydraulic Fracturing in the oil andnatural gas production, no filling materials (“proppants”) to keep thefissures open are hereby necessary in many cases, because the highpressure is maintained during the operation.

In geothermal drilling processes, it is typically required to pump in adrilling fluid into the geothermal borehole. This drilling fluid (aprocess fluid) is usually a suspension of ground bentonite in water withfurther additives. The drilling fluid is usually kept in a continuouscycle. Among others, it usually serves for stabilising the borehole, forexporting the drilling debris, and for discharging the frictional heatcaused by the drilling tools.

The interior of the earth's crust has been colonised or is colonisableby numerous species of microorganisms, both in the ground and in theunderlying geological formations. Due to their growth and metabolism,many of these microorganisms may complicate or even make impossiblegeothermics, mainly deep geothermics, and in particular the geothermaldrilling process; i.e. these are “undesirable microorganisms”.Undesirable microorganisms may, for example, promote corrosion ofconducting tubes and of other equipment made of metal by their metabolicproducts and may clog pipes by forming a slime (i.e. formation ofextracellular polymeric substances) or by their growth (“biofilms”).This is also known as “biofouling”. In addition, undesirablemicroorganisms may accumulate in a process fluid without a biostabiliserand may endanger ecological systems and human beings in case thisprocess fluid should leak out of a controlled setting into theenvironment (e.g. into groundwater).

On these and other grounds, biocides or biostabilisers (also referred toas biostats or biostatic agents) are frequently employed againstundesirable microorganisms.

Usually, aggressive biocides such as glutaraldehyde or triazinederivatives are used in geothermics, mainly in deep geothermics, and inparticular in the geothermal drilling process, which are harmful tohumans and the environment. This problem has been recognised, asdescribed e.g. in Ashraf et al., Environmentally compatible biocides(“green biocides”), on a general term. In particular, the followingpublications disclose alternatives to traditional biocides for use inthe oil and gas production:

However, such alternative biocides of the prior art have one or moredisadvantages, such as: higher costs, lower efficiency, especially underthe conditions which may prevail in the Earth's crust (e.g. highertemperatures), and for many, especially thermophilic microorganisms,complexity of application. In addition, these are for their most partstill biocides and no biostabilisers which may have unclear effects onecosystems. Furthermore it is advantageous to have as many different andpreferably environmentally compatible biocides or biostabilisersavailable as possible to collectively achieve a broad spectrum ofactivity against numerous species of undesired microorganisms. The issueof environmental compatibility is most notably in Europe of particularimportance, and in fact even a prerequisite for further establishinggeothermics, in particular deep geothermics. It is further believed thatthe effectiveness of the previously used biocides is reduced, amongothers, by the high salt concentrations which sometimes occur when, forexample, a process fluid used in the Earth's crust detaches salt fromthis Earth's crust.

For these and other reasons, one object of the present invention is toprovide a process fluid comprising an environmentally compatiblebiostabiliser for geothermics (in particular for deep geothermics), i.e.in a geothermal borehole, and in front of all for a geothermal drillingprocess, i.e. in this case in the form of a drilling fluid, as well as arelated production method and a corresponding method of use. Inparticular, this biostabiliser is to be effective against selectedundesirable mesophilic or thermophilic microorganisms; especially thosewhich are prevented only insufficiently from growth or metabolism by thecurrently used biocides. Specifically, this biostabiliser should beeffective in an environmental condition typical of the Earth's crust,especially of geothermally utilisable geological formations.Furthermore, this biostabiliser should be producible and employable assimply and economically as possible because this is required for itsindustrial scale use.

Accordingly, the present invention relates to a process fluid for ageothermal bore (and the use of the process fluid in a geothermalborehole). The process fluid of the invention comprises a biostabiliserand is characterised in that the biostabiliser is comprised of at leastone organic acid or a salt, alcohol or aldehyde thereof, wherein the atleast one organic acid is selected from the group consisting of hopacids, resin acids, fatty acids and mixtures thereof.

Surprisingly, these organic acids have been found particularly suitablefor biostabilisation in geothermics, in particular in deep geothermics.

Thus, these organic acids are also effective in particular at the highertemperatures which may prevail in geological formations in a depth of 1km to 5 km, and even against selected unwanted mesophilic orthermophilic microorganisms which can occur or grow in this environment.

These organic acids may be added particularly simply andcost-effectively to obtain the process fluid of the invention—forexample, hop acids may be added in the form of a hop extract andselected resin acids may be added in the form of a natural resin,especially rosin—preferably as an alkaline solution of selected resinacids in the form of a natural resin, especially rosin.

Said organic acids have already been proven to be biostabilising in thefood production, as described, inter alia, in the documents WO 00/053814A1, WO 01/88205 A1, WO 2004/081236 A1 and WO 2008/067578 A1. The use ingeothermics or even in deep geothermics, is, however, not suggested inthese documents. In use in food production, these organic acids havebeen found to be well tolerated by humans and the environment.

In Emerstorfer et al. the minimum inhibitory concentration of hop betaacids, resin acids and a mixture of resin acids and myristic acid wasinvestigated against various bacteria, yeasts and moulds and wascompared to the effect of potassium hydroxide and hydrogen peroxide. Theuse in geothermics or even in deep geothermics is, however, notsuggested in this document.

Although Wang et al. refers to resin acid derivatives as antimicrobialagents, too, but their use in geothermics or even in the Earth's crust,especially in the difficult conditions typical of deep geothermics, isalso not suggested in this document.

The subject of US 2003/0015480 A1 is a method using hop acid to control(the growth) of (micro)organisms, for example in the paper production.Uses in geothermics are, however, neither disclose nor suggested.

GB 1 417 237 discloses a drilling mud on an aqueous base. Therein, atall oil fraction having a high content of resin acid is proposed, butthere is no mention of a biostabilising effect or even of a geothermalbore. The range of concentrations for the tall oil fraction representedas being essential for its lubricity is between 0.45% and 3% (v/v), i.e.substantially higher than the range preferred for the biostabilisingeffect of between 0.25 and 500 ppm.

US 2015/353806 relates to concentrates to be added into drilling fluidsto improve the lubricity of them. Among others, “the acids [ . . . ] ofresin acids” are mentioned as feasable contents of the concentrate toimprove the lubricity but even in this document there is no mention ofeither a biostabilising effect or a geothermal bore. None of thedocuments mentioned anticipates the present invention nor does any ofthem lead to it.

Its use in geothermics requires large amounts (volumes) of the processfluid of the invention. Preferably, the process fluid of the inventionis thus provided in an amount of at least 10⁴ L, preferably at least 10⁵L, specifically at least 10⁶ L. In a geothermal drilling process, forexample, from 10⁵ to 10⁷ liter of the process fluid of the presentinvention are usually required as a drilling fluid.

The crust is the outermost solid shell of the earth and may extent to adepth of approximately 100 km, on average approximately to a depth of 35km. The top layer of the Earth's crust ususally forms the ground, with ausual depth of about 10 m-20 m. Below lies a wide variety of geologicalformations with different widths. Generally, the temperature of theEarth's crust increases with every kilometer depth by about 25° C.-30°C., with considerable local deviations.

For example, the geothermal deposits which are to be mined by deepgeothermics are often at a depth of 1-3 km, so that the temperaturethere may usually be 25°-90° C.; thus, depending on local conditions,mesophilic, thermophilic and/or hyperthermophilic microorganisms areplaying a role especially in deep geothermics. In one kilometer ofdepth, temperatures of more than 50° C., a pore pressure of more than 30MPa and pH values of less than 6 are the environmental conditions to beexpected there. Depending on local conditions, halophilic orhalo-tolerant microorganisms are playing a role especially in deepgeothermics, too.

In addition to the microorganisms resident in their respective depths,the growth of them has sometimes to be combatted, which were, forexample, introduced by the pumping of the process fluid itself into therespective depth.

In the following, the microorganisms undesirable especially in deepgeothermics will be described: the undesirable microorganisms areselected from the group of bacteria, fungi and archaea, preferably theyare selected from the group of bacteria. Particularly undesirable aremicroorganisms (in front of all bacteria) which produce one or more ofthe following: acid, extracellular polymeric substances (e.g. inbiofilms), and sulphides. Particularly undesirable are sulphideproducers, among others because of the resulting odor, health concerns,and the corrosion caused by the resulting sulphides

Preferably, the undesired microorganisms are bacteria. They preferablybelong to the phylum of Firmicutes, Bacteroidetes, Actinobacteria orProteobacteria, especially to the phylum of Firmicutes. They preferablyalso belong to one of the following genera: Pseudomonas, Cobetia,Shewanella, Thermoanaerobacter, Arcobacter, Pseudoalteromonas,Marinobacterium, Halolactibacillus (also known as Halolactobacillus),Selenihalanaerobacter, Vibrio, Desulfovibrio, Burkholderia, Arcobacter,Dietzia, Microbacterium, Idiomarina, Marinobacter, Halomonas andHalanaerobium, more preferably to a genus selected fromHalolactibacillus and Halanaerobium.

Out of the archaea, particularly undesired are the generaMethanosarcinales, Methanohalophilus and Methanolobus.

It has been found as part of the invention that the process fluid withthe inventive biostabiliser is effective against many of these undesiredmicroorganisms, preferably against many of the firmicutes oractinobacteria, especially against sulphide producing firmicutes. It wasparticularly surprising that the inventive process fluid with thebiostabiliser was effective against bacteria selected from the generaDietzia, Microbacterium, Halolactibacillus and Halanaerobium.

The inventive process fluid is particularly effective against members ofHalolactibacillus. It can be assumed that conventional environmentallyharzardous biocides-unlike the biostabiliser according to theinvention—are insufficiently effective against Halolactibacillus.

The genus Halolactibacillus includes, for example, H. halophilus and H.miurensis. The biostabiliser according to the present invention isparticularly effective against both of these types.

The biocides used hitherto act only insufficiently especially againstHalanaerobium. The genus Halanaerobium includes for exampleHalanaerobium congolense, which grow well for example at a massconcentration of 10% NaCl and 45° C. under anaerobic conditions andwhich can reduce thiosulphate or sulphur compounds to sulphides whichmay result in an undesirable odor development. Also Halanaerobiumpraevalens is particularly undesirable.

Said organic acids or constituents which contain said organic acids areknown per se from, inter alia, the documents WO 00/053814 A1, WO01/88205 A1, WO 2004/081236 A1 and WO 2008/067578 A1. All productionprocesses or preparation methods described in these documents,especially for hop extract, natural resin or myristic acid, or a saltthereof, are preferred according to the present invention.

Hop acids are ingredients of unfertilised blossoms of female hop plants.These bitter-tasting hop ingredients have been used for the productionof storable beer for centuries and have, thus, even found their way intohuman nutrition. The environmental compatibility, especially at thefinal concentrations proposed herein, is thus given.

The hop plant Humulus lupulus belongs to the botanical family ofCannabaceae; hop is cultivated in many countries and used for theproduction of beer. Unfertilised female hop plants form the so-calledhop cones which are holding the hop resin. Hop resin, in turn, containsthe most varying kinds of biostabilising substances. The hop ingredientscan be extracted using ethanol or supercritical CO₂.

The bitter constituents recoverable from the hop resin include variousfractions such as humulone (alpha acid) and lupulone (beta acid). Thesesubstances have microbiological inhibitory activity and can be convertedinto their isoforms by heating, whereby better water solubility is givenat a still existing inhibitory effect on undesired microorganisms. Toincrease the solubility and prevent precipitation at storage, it issometimes favorable to add myristic acid in small amounts as a technicalexcipient already in the preparation process. Examples of suitable hopacids can also be found in WO 00/053814 A1.

Even many fatty acid compounds are physiologically harmless naturalproducts. The environmental compatibility particularly in the finalconcentrations suggested herein is thus given. The fatty acid compoundsaccording to the present invention may also be fatty acid alcohols orfatty acid aldehydes. The fatty acid compounds may also be modified suchas by the incorporation of functional groups such as —OH, —SH, —NH₂, —F,—Cl, —Br, —I, and the like (except derivatives which are toxic tohumans, animals or plants); aliphatic side chains and/or one or more(especially two or three) (unsaturated) double bonds are possible aswell, as long as the physico-chemical properties of the (aliphatic)backbone, in particular the solubility in biostabilising concentrationsas well as the structure of the C1 atom are preserved. Thebiostabilising effect of fatty acids is known for example from WO2004/081236 A1. In general, experiments have shown that in general thefree fatty acids and their soaps according to the present invention havebetter antimicrobial efficacy than their aldehydes or esters. Inparticular myristic acid or its soap has proven particularly useful inthe invention, especially with respect to its antimicrobial activity.

Tree resins from pine, for example, and the rosin obtained therefrom bydistillation which consists mostly of resin acids, have bactericideproperties which have been used for human consumption for centuries. Theenvironmental compatibility, especially at the final concentrationsproposed herein, is thus given.

Preferably, the resin acids or the resin are obtained from pines. Pines,such as the austrian black pine Pinus nigra Austriaca, belong to thebotanical family of Pinaceae; they are primarily widespread in thenorthern hemisphere and the resins therefrom have a long tradition inthe production of Retsina, a Greek resinated wine. To obtain thebiostabilisingly active ingredients the pine resin is preferablyseparated by distillation into the two fractions turpentine and rosin.Rosin now consists of the active ingredients which are a mixture ofresin acids. These can be dissolved with alkalis to alkaline resinates.

Rosin is a mixture of aromatic compounds such as abietic acid,dehydroabietic acid and isomers thereof. These so-called resin acids,which are commercially available in the form of solid rosin blocks, havea biostabilising activity of varying degrees and can be used as watersoluble alkali resinates. To increase the solubility and preventprecipitation of the resin soap at storage it is favorable to addmyristic acid in small amounts as a technical exzipient already in itspreparation process.

According to the invention, all food-compatible resins, such asdescribed for example in “Ullmann's Encyclopedia of IndustrialChemistry”, Vol. A 23 (1993) pages 73-88, can be used, such as woodresins, more particular balms, such as benzoin, pine balm, myrrh andtolu balm. For reasons of economy and in accordance with the invention,mainly rosin products and their derivatives are preferred. Products likethis are described for example in Ullmann's Encyclopedia of IndustrialChemistry, Vol. A 23 (1993) pages 79-88.

Partly because of the above-described surprising biostabilising effectin terms of selected mesophilic, thermophilic, hyperthermophilic,halo-tolerant and/or halophilic bacteria, in particular of the phylusFirmicutes or Actinobacteria, is the process fluid of the presentinvention in a preferred embodiment provided for use in a geothermalborehole in deep geothermics. In particular, the process fluid of theinvention is provided for a geothermal borehole in a Hot DryRock-process.

In the course of the present invention it has been shown that theinventive process fluid acts particularly well biostabilising withrespect to the unwanted microorganisms mentioned herein, whenbiopolymers such as polysaccharides, especially starch and modifiedstarches, are included as a gelling agent. These biopolymers are veryvulnerable to microbial decomposition for example in the geothermalborehole. On the other hand, an (intentional) decomposition of thecomponents added to the process fluid such as the biopolymers and/or theinventive biostabilisers may be desirable after a certain time. Both canbe realised with the present invention, because the degradation of thesesubstances—due to its harmlessness in principle and its fundamentallysafe biodegradability—can be controlled to a certain extent.

Therefore, the process fluid of the invention is characterised in apreferred embodiment in that it further contains at least one gellingagent, wherein the gelling agent is a biopolymer or a polymericderivative thereof; preferably, wherein the biopolymer is apolysaccharide, preferably a starch, a vegetable gum such as xanthan, acellulose, in particular a polyanionic cellulose, or a pectin,especially a starch. According to the invention, any derivatisation ofthe biopolymer deemed appropriate by one of skill in the art is, amongother things, possible in this embodiment; however, the gellingproperties of the biopolymer must essentially be retained or—withrespect to degradability—must be adjustable or controllable. Preferredbiopolymers or derivatives thereof are also disclosed in WO 2012/045711A1, U.S. Pat. No. 4,659,811, WO 2006/109225, U.S. Pat. No. 5,681,796,U.S. Pat. No. 4,964,604, U.S. Pat. No. 4,169,798 or U.S. Pat. No.6,810,959, or in selected ones of the above quoted documents.

In a further preferred embodiment of the invention the inventive processfluid is used as a drilling fluid in a geothermal borehole. Theinventive process fluid has proven to be particularly suitable for thispurpose (see Example 1B).

In the process fluid of the invention further components can becontained, among others components which are typical for a drillingfluid, or another use, in geothermics or deep geothermics, or any othercomponents which one of skill in the art might deem expedient for use ina geothermal borehole. Preferably, one or more substances are presentselected from the following groups: gelling agents, in particular thosementioned two paragraphs further above; buffering agents, in particularthose selected from acetic acid, fumaric acid, potassium carbonate,borax, sodium acetate, sodium bicarbonate, sodium carbonate, sodiumhydroxide; and clay minerals, in particular bentonite, in a finelygranulated form (e.g. ground). Example 1A shows an inventive processfluid, which was successfully used as a drilling fluid in a geothermalborehole.

Surprisingly it has been found that it is advantageous if a defoamer iscontained in the inventive process fluid, in particular when it is usedas a drilling fluid (see Example 1B). Preferably, this defoamer is basedon non-ionic surfactants, for example on fatty alcohol alkoxylate oralkylene oxide polymer basis. A suitable product is for example BASOPUR®DF 5 of BASF SE.

Preferably, the process fluid of the present invention comprises water.

Surprisingly it has been found in the course of the present inventionthat the process fluid of the invention, if water is contained or isadded, acts biostabilising in regard to the undesirable microorganismsmentioned herein the better, the softer the water is or will be made.The inventive process fluid acts particularly well biostabilising whenthe water hardness is at most 20° dH (German Hardness) or 3.57 mmol/l(alkaline earth ions), preferably not more than 15° dH or 2.67 mmol/l,more preferably at most 10° dH or 1.78 mmol/l, even more preferred atmost 7.5° dH or 1.34 mmol/l, in particular at most 5° dH or 0.89 mmol/l.By the inventive use of the process fluid in particular in deepgeothermics alkaline earth ions can be brought into solution, resultingin a higher water hardness.

Therefore, the process fluid of the present invention is in a preferredembodiment characterised in that it further comprises a water softener.All water softener which deem expedient to one skilled in the art areappropriate. Preferably, the water softener is a cation exchanger or achelator, in particular selected from: zeolites (such as zeolite A),inorganic polyphosphates (such as triphosphate), ethylenediaminetetraacetic acid and salts thereof, nitrilotriacetic acid and saltsthereof, polyacrylates, and citrate (or citric acid).

It has been found that the inventive process fluid acts particularlywell biostabilising in regard to the undesirable microorganismsmentioned herein if a plurality of the named organic acids is containedtherein. This results in a synergistic effect with respect to thebiostabilising effect. Therefore, another preferred embodiment of thepresent invention relates to the inventive process fluid, furthercharacterised in that the biostabiliser comprises a mixture, which ispreferably selected from:

at least one hop acid, or a salt, alcohol or aldehyde thereof, and atleast one fatty acid, or a salt, alcohol or aldehyde thereof, or

at least one resin acid, or a salt, alcohol or aldehyde thereof, and atleast one fatty acid, or a salt, alcohol or aldehyde thereof, or

at least one hop acid, or a salt, alcohol or aldehyde thereof, and atleast one resin acid, or a salt, alcohol or aldehyde thereof, and atleast one fatty acid, or a salt, alcohol or aldehyde thereof;

in particular wherein the biostabiliser is a mixture of at least one hopacid, or a salt, alcohol or aldehyde thereof, and at least one resinacid, or a salt, alcohol or aldehyde thereof, and at least one fattyacid, or a salt, alcohol or aldehyde thereof.

In the course of the present invention, one biostabiliser hassurprisingly proven particularly effective in regard of the undesiredmicroorganisms mentioned herein, comprising at least one selected fromhop extract, a natural resin (especially rosin) and myristic acid or asalt thereof. Consequently, the process fluid of the present inventionis in a particularly preferred embodiment characterised in that thebiostabiliser is a mixture of at least one, preferably at least two, inparticular all of the following components: hop extract, natural resin,preferably rosin, wherein the natural resin is preferably added indissolved form, and myristic acid or a salt thereof. Anotherparticularly preferred embodiment relates to the process fluid accordingto the invention, which is further characterised in that thebiostabiliser is obtainable by adding at least one, preferably at leasttwo, in particular all of the following components: hop extract, naturalresin, preferably rosin, wherein the natural resin is preferably addedin dissolved form, and myristic acid or a salt thereof.

Preferably, the hop acid of the present invention consists in an alphahop acid, selected from the group consisting of humulone, isohumulone,cohumulone, adhumulone, prehumulone, posthumulone,tetrahydroisohumulone, and tetrahydrodeoxyhumulone, or a beta hop acid,selected from the group consisting of lupulone, colupulone, adlupulone,prelupulone, postlupulone, hexahydrocolupulone, and hexahydrolupulone,because these are suitable for biostabilisation.

Preferably the resin acid of the present invention is selected from thegroup consisting of pimaric acid, neoabietic acid, abietic acid,dehydroabietic acid, levopimaric acid, and palustrinic acid, becausethese are suitable for biostabilisation.

Preferably the fatty acid of the present invention is selected from thegroup consisting of capric acid, undecylenic acid, lauric acid, myristicacid, palmitic acid, margaric acid, stearic acid, arachinic acid,behenic acid, lignoceric acid, cerotic acid, palmitoleinic acid, oleicacid, elaidic acid, vaccenic acid, icosenoic acid, cetoleic acid, erucicacid, nervonic acid, linoleic acid, linolenic acid, arachidonic acid,timnodonic acid, clupanodonic acid, and cervonic acid, because they aresuitable for biostabilisation. Particularly preferred is myristic acid.

Investigations in the course of the present invention have come toconcentration ranges for the biostabiliser's components which areparticularly suitable for biostabilisation regarding undesirablemicroorganisms. Thus, another preferred embodiment of the presentinvention refers to an inventive process fluid which is characterised inthat:

the total concentration of hop acids in the process fluid is 0.01-1000ppm, preferably 0.05-100 ppm, more preferably 0.1-10 ppm, particularly0.5-5 ppm; and/or

the total concentration of resin acids in the process fluid is 0.05-5000ppm, preferably 0.25-500 ppm, more preferably 0.5-50 ppm, particularly0.25-25 ppm; and/or

the total concentration of fatty acids in the process fluid is 0.05-5000ppm, preferably 0.25-500 ppm, more preferably 0.5-50 ppm, particularly0.25-25 ppm.

Preferably, “ppm” (“parts per million”) refers to the percentage of eachorganic acid(s) (in mg) of the total mass of the process fluid (in kg),i.e. ppm stands for mg/kg.

The stated concentrations are final biostabiliser concentrations, i.e.the process fluid can be pumped with the mentioned final concentrationsof the biostabiliser into the geothermal borehole and can there unfoldits biostabilising effect, in addition to the technical result for eachspecific application. The process fluid is compatible with theenvironment in particular with these final concentrations.

In selected situations, it is necessary to enlarge the biostabilitoryeffective spectrum of the process fluid of the present invention or toprovide additional biocide effects therewith. Therefore, the inventiveprocess fluid is in a further preferred embodiment characterised in thatit further comprises at least one other microbially active substanceand/or biostabiliser, preferably selected from acetic acid, lactic acid,propionic acid, benzoic acid, sorbic acid, formic acid, and saltsthereof; chitosan and chitosan derivatives, such as disclosed in WO2012/149560 A2, are preferred, too.

In a further aspect of the present invention, the use of the inventiveprocess fluid in a geothermal borehole, preferably for deep geothermics,in particular for the Hot Dry Rock process, is disclosed. In a preferredembodiment, at least 10⁴ L, preferably at least 10⁵ L, in particular atleast 10⁶ L of the inventive process fluid are used. Preferably, anundesirable microorganism is inhibited in its growth and/or metabolismby the biostabiliser of the process fluid, which is a bacteria,preferably selected from the phylus of Firmicutes, Actinobacteria,Bacteroidetes, or Proteobacteria, in particular of the phylus ofFirmicutes or Actinobacteria. This undesirable microorganism furtherbelongs to a genus of bacteria selected from Pseudomonas, Cobetia,Shewanella, Thermoanaerobacter, Arcobacter, Pseudoalteromonas,Marinobacterium, Halolactibacillus, Selenihalanaerobacter, Vibrio,Desulfovibrio, Burkholderia, Arcobacter, Dietzia, Microbacterium,Idiomarina, Marinobacter, Halomonas and Halanaerobium, more preferablyto a genus selected from Dietzia, Microbacterium, Halolactibacillus andHalanaerobium.

In a further preferred embodiment the inventive process fluid is used ina depth of 100 m-8000 m, preferably of 200 m-7000 m, more preferably of300 m-6000 m, even more preferred of 400 m-5000 m, particularly of 500m-4000 m or even of 600 m-3500 m.

The inventive process fluid is preferably employed for use as a drillingfluid.

Another aspect of the present invention relates to a method forbiostabilising a geothermal borehole, preferably for deep geothermics,in particular for a Hot Dry Rock process, comprising pumping the processfluid into the borehole. In a preferred embodiment at least 10⁴ L,preferably at least 10⁵ L, in particular at least 10⁶ L of the inventiveprocess fluid are pumped into the geothermal borehole.

In a further preferred embodiment of the process for biostabilising ageothermal borehole, the inventive process fluid is used in a depth of100 m-8000 m, preferably of 200 m-7000 m, more preferably of 300 m-6000m, even more preferred of 400 m-5000 m, particularly of 500 m-4000 m oreven of 600 m-3500 m in the geothermal borehole. In this process, theinventive process fluid is preferably used as a drilling fluid.

Preferably, in the inventive process for biostabilisation, an unwantedmicroorganism is inhibited in its growth and/or metabolism by thebiostabiliser of the process fluid, which is a bacteria, preferablyselected from the phylus of Firmicutes, Actinobacteria, Bacteroidetes orProteobacteria, in particular of the phylus of Firmicutes orActinobacteria. This undesirable microorganism further belongs to agenus of bacteria selected from Pseudomonas, Cobetia, Shewanella,Thermoanaerobacter, Arcobacter, Pseudoalteromonas, Marinobacterium,Halolactibacillus, Selenihalanaerobacter, Vibrio, Desulfovibrio,Burkholderia, Arcobacter, Dietzia, Microbacterium, Idiomarina,Marinobacter, Halomonas and Halanaerobium, more preferably to a genusselected from Dietzia, Microbacterium, Halolactibacillus andHalanaerobium. In a further aspect of the present invention, a methodfor preparing the inventive process fluid is disclosed, wherein theprocess fluid comprises water. The method comprises adding at least oneorganic acid or a salt, alcohol or aldehyde thereof, to water or awater-containing portion of the process fluid, wherein the at least oneorganic acid is selected from the group consisting of hop acids, resinacids, fatty acids, and mixtures of two or all of them. In a preferredembodiment at least 10⁴ L, preferably at least 10⁵ L, in particular atleast 10⁶ L of the inventive process fluid are prepared.

If the inventive process fluid comprises a resin acid, it is extremelyeconomical to add it in the form of a resin or a distillate thereof,especially rosin. Consequently, the preparation method of the inventionis, if at least one resin acid will be supplied, characterised in apreferred embodiment in that the at least one resin acid is added in theform of a resin, preferably of a natural resin, even more preferred inthe form of rosin. In another embodiment it is favorable to employ indoing so a dissolved, emulsified or dispersed, or pasty, rosin product,preferably based on pine resin, pine balm, rosin acids, salts of rosinacids (resin soaps), non-denatured derivatives of pine resins (i.e.derivatives, obtained without the influence of strong acids or bases).As rosin derivatives, preferred according to the invention are alsoindividual components of rosin which are either chemically synthesisedor isolated from rosin products, such as levopimaric acid, neoabieticacid, palustrinic acid, abietic acid, dehydroabietic acid,tetrahydroabietic acid, dihydroabietic acid, pimaric acid, andisopimaric acid. Derivatisation of rosin may within the meaning of theinvention further provide for hydrogenation, polymerisation, additionreactions, esterification, nitrilation, amination etc. In theembodiments described in this paragraph it is particularly preferred ifthe respective resin acid containing component (such as the resin or adistillate thereof) is added as an alcoholic solution or suspension,preferably as a 1 to 95%, especially as a 10 to 80% solution of ethanol,or as an alkaline solution, preferably a 0.5 to 35% alkaline solution(the at least one fatty acid may also be added in an alkaline solutionas just described).

It may also be expedient in the preparation process according to theinvention to add, if at least one resin acid and/or fatty acid is to beadded, this at least one resin acid and/or fatty acid as a salinesolution or suspension, preferably as a potassium salt solution,particularly as a 0.5 to 35% potassium salt solution.

For economic reasons, it is preferable to add, if at least one hop acidis to be added in the preparation process, this at least one hop acid inthe form of a hop extract. The production of hop extract itself has beenknown for long, and usually the extraction from unfertilised blossoms offemale hop plants is effected with alcohol, especially ethanol, as asolvent, or preferably by extraction with supercritical CO₂. Otherpreferred variants of the addition of hop extract are disclosed in WO00/053814 A1.

In a further preferred embodiment of the present invention, thepreparation process of the invention further comprises the addition of afurther antimicrobial agent or of stabiliser to water or awater-containing portion of the process fluid, preferably selected fromacetic acid, lactic acid, propionic acid, benzoic acid, sorbic acid,formic acid and salts thereof. The addition of chitosan and chitosanderivatives, such as disclosed in WO 2012/149560 A2, is also preferred.

Further preferred features of the preparation process according to theinvention are disclosed in the documents of WO 00/053814 A1, WO 01/88205A1, WO 2004/081236 A1 or WO 2008/067578 A1.

The inventive process fluid may for example be a liquid, a gel or aliquid foam. Preferably, the process fluid is water. Preferably, theprocess fluid is a liquid or a gel.

Furthermore, it is preferable that the biostabiliser of the processfluid of the invention is dissolved in particular in water or the watercontaining portion of the process fluid—however, the biostabiliser may,partly or wholly, be provided in suspension or emulsion in the processfluid of the invention.

A biostabiliser is understood to be a substance which may slow down orinhibit the growth (in particular the proliferation) and/or themetabolism of microorganisms such as bacteria, archaea or fungal cells.In contrast, a biocide is understood to be a substance which killsmicroorganisms. In this regard, a biocide acts more aggressive than abiostabiliser. To those skilled in the art it is evident that abiostabiliser may also act as a biocide under special circumstances(e.g. at high doses in highly susceptible microorganisms etc.).

The biostabilising effect of a substance can be measured in manydifferent ways, which are known in the art, among others by means ofmethods such as are disclosed in the documents cited herein. Preferredmethods for determining the biostabilising effect are the microdilutiontest, spot test or well diffusion test. Also, the methods disclosed inWhite et al. or in Jorgensen and Ferraro are preferred.

For the measurement of the biostabilising effect, the substance is oftenadded to a sample up to one or more particular concentrations (e.g. 0.1ppm, 0.5 ppm, 1 ppm, 5 ppm, 10 ppm, 25 ppm, 50 ppm, 100 ppm, 200 ppm,300 ppm, 400 ppm, 500 ppm, 1000 ppm, 5000 ppm, 10000 ppm), wherein thesample comprises the living undesirable microorganism and may, forexample, be a pure culture, a mixed culture, a sample taken from theEarth's crust or a sample which is similar to a sample taken from theEarth's crust (e.g. sewage sludge). Then the biostabilising effect ofthe substance can be determined by comparing the sample with thesubstance after one or more certain time periods (e.g. 1 d, 2 d, 3 d, 4d, 5 d, 10 d, 20 d, 30 d, 60 d, 90 d, 120 d) with itself at the initialtime point and/or with a control (the sample without the substance, e.g.with water instead of the substance). This comparison may be a directcomparison, including: determining the number of bacteria by plating anddetermining the CFU/ml (colony forming units) or measuring the turbidity(e.g. determining the OD600). This comparison may as well be an indirectcomparison, e.g. the measurement of an undesired effect which may becaused by the undesirable microorganism (e.g. sulphide production, acidproduction, production of extracellular polymeric substances).Particularly in deep geothermics, a reduced microbial sulphideproduction can be an important parameter to determine the suitability ofthe substance as a biostabiliser. A biostabiliser preferably has one ormore of the described effects on selected undesirable microorganisms,such as those mentioned herein: Lower increase in biomass than in thecontrol, lower sulphide production than in the control, lower acidproduction than in the control, lower production of extracellularpolymeric substances than in the control, lower biofilm production thanin the control.

According to Brock Biology of Microorganisms, page 138, the termsmesophilic, thermophilic, hyperthermophilic, halo-tolerant, andhalophilic are to be understood as follows:

Mesophilic: refers to a microorganism which grows best between 20° C.and 45° C.; thermophilic: refers to a microorganism which grows bestbetween 45° C. and 80° C.; hyperthermophilic: refers to a microorganismwhich grows best at 80° C. and higher; halo-tolerant: refers to amicroorganism which can grow in high salt concentrations (e.g. a massconcentration of 25% NaCl); halophilic: refers to a microorganism whichneeds high salt concentrations (e.g. a mass concentration of 25% NaCl)for growth.

The present invention is further illustrated by the following figuresand examples, to which it will of course not be limited.

FIGS. 1A-1M: Effect of the biostabilisers on Halanaerobium congolense.In accordance with example 4, the strain DSM 11287 was exposed tobiostabiliser A (hop acid) or to biostabiliser B (resin acid/myristicacid) in various concentrations. Shown is the (A) growth curve withoutbiostabiliser; growth curve at (B) 0.5 ppm, (C) 1 ppm, (D) 10 ppm, (E)50 ppm, (F) 100 ppm, (G) 250 ppm of biostabiliser A; and growth curve at(H) 0.5 ppm, (I) 1 ppm, (J) 10 ppm, (K) 50 ppm, (L) 100 ppm, (M) 250 ppmof biostabiliser B. The dose-dependent tendency towards biostabilisationis clearly evident.

FIGS. 2A-2M: Effect of the biostabilisers on Halolactibacillusmiurensis. In accordance with example 4, the strain DSM 17074 wasexposed to biostabiliser A (hop acid) or to biostabiliser B (resinacid/myristic acid) in various concentrations. Shown is the (A) growthcurve without biostabiliser; growth curve at (B) 0.5 ppm, (C) 1 ppm, (D)10 ppm, (E) 50 ppm, (F) 100 ppm, (G) 250 ppm of biostabiliser A; andgrowth curve at (H) 0.5 ppm, (I) 1 ppm, (J) 10 ppm, (K) 50 ppm, (L) 100ppm, (M) 250 ppm of biostabiliser B. The dose-dependent tendency towardsbiostabilisation is clearly evident.

FIGS. 3A-3M: Effect of the biostabilisers on Halolactibacillushalophilus. In accordance with example 4, the strain DSM 17073 wasexposed to biostabiliser A (hop acid) or to biostabiliser B (resinacid/myristic acid) in various concentrations. Shown is the (A) growthcurve without biostabiliser; growth curve at (B) 0.5 ppm, (C) 1 ppm, (D)10 ppm, (E) 50 ppm, (F) 100 ppm, (G) 250 ppm of biostabiliser A; andgrowth curve at (H) 0.5 ppm, (I) 1 ppm, (J) 10 ppm, (K) 50 ppm, (L) 100ppm, (M) 250 ppm of biostabiliser B. The dose-dependent tendency towardsbiostabilisation is clearly evident.

FIGS. 4A-4C: Effect of hop acids compared to the chemical biocidemethylenbis[5-methyloxazolidine] on Halolactibacillus miurensis. Inaccordance with example 5, the strain DSM 17074 was exposed to a hopacid containing hop extract or to the biocide3,3′-methylenbis[5-methyloxazolidine] known in the art in variousconcentrations. Shown is the (A) growth curve without biostabiliser; (B)growth curves at 10, 25 or 50 ppm methylenbis[5-methyloxazolidine]; (C)growth curves at 10, 25 or 50 ppm hop acids. The dose-dependent tendencytowards biostabilisation is clearly evident. Furthermore, it can be seenfrom the figures that the biostabilising effect of hop acids onHalolactibacillus miurensis is surprisingly stronger even at lowerconcentrations of for example 10 ppm than in the case of the chemicalbiocide methylenbis [5-methyloxazolidine].

FIGS. 5A-5C: Effect of hop acids compared to the chemical biocidemethylenbis[5-methyloxazolidine] on Halolactibacillus halophilus. Inaccordance with example 5, the strain DSM 17074 was exposed to a hopacid containing hop extract or to the biocide3,3′-methylenbis[5-methyloxazolidine] known in the art in variousconcentrations. Shown is the (A) growth curve without biostabiliser; (B)growth curves at 5, 10, 25, 50, 100, 200, 1000, 2000 or 5000 ppmmethylenbis[5-methyloxazolidine]; (C) growth curves at 0.25, 0.5, 20,100 or 200 ppm hop acids. The dose-dependent tendency towardsbiostabilisation is clearly evident. Furthermore, it can be seen fromthe figures that the biostabilising effect of hop acids onHalolactibacillus halophilus is surprisingly stronger even at lowerconcentrations of for example 20 ppm than in the case of the chemicalbiocide methylenbis[5-methyloxazolidine] at for example 25 ppm.

EXAMPLES Example 1A

Preparation of the inventive process fluid as a drilling fluid for ageothermal borehole For a geothermal borehole, 750000 L of process fluidwith a biostabiliser were provided as a drilling fluid:

The following substances were added to 720000 L of water: hop acidextract as a biostabiliser (700 kg of a 10% alkaline hop acid solutionfor a hop acid concentration of 1 g/l). 61000 kg potassium carbonate toinhibit drilled solids; 18000 kg polyanionic cellulose (PAC) and 2250 kgxanthan.

In addition, the following substances were added: 4000 kg of citricacid, 1500 kg of soda, 3000 kg of bentonite and 720 L of defoamer on afatty alcohol oxylate base.

Example 1B

Inventive use of the process fluid as a drilling fluid in a geothermalborehole

When using a drilling fluid with the biostabiliser of Example 1A at ageothermal borehole in a drilling depth of 750-3200 m, microbiologicalcontamination has been significantly reduced and the adverse effectssuch as odor, change in viscosity of the drilling fluid or degradationof xanthan can be prevented.

The microbiological tests were carried out on platecount agar by plating100 μl of a drilling fluid sample and incubating for two days at 37° C.(the microbiological load is indicated in CFU=colony forming units perml drilling fluid):

Day 1 Start of the second bore section (750 m depth). Drilling fluid ofExample 1A, but without biostabiliser and defoamer, was used

Day 11 Sampling from drilling fluid—bacterial growth overgrown agar, CFUtherefore not well defined but surely far more than 3000. Among otherthings, a significant proportion of bacteria of the generaMicrobacterium and Dietzia was present in the sample, as determined bysequencing. The drilling fluid of Example 1A with biostabiliser, butwithout defoamer, was now used. Unexpectedly it was shown that the useof a defoamer was advantageous so that after a short time the drillingfluid of Example 1A (i.e. with biostabiliser and defoamer) was used.

Day 18 >300 CFU/ml  Day 21 93 CFU/ml Day 29 13 CFU/ml Day 37 14 CFU/mlDay 43 19 CFU/ml Day 50 18 CFU/ml Day 61 End of drilling

Thus, it has surprisingly been found that the process fluid with thebiostabiliser according to the invention is also effective as a drillingfluid in a geothermal drilling, particularly against bacteria of thegenera Microbacterium and Dietzia.

Example 2

Biostabilising Effect on Halanaerobium

Preparation of the Growth Medium:

Trace element stock solution: Add 1.50 g of nitrilotriacetic acid to 1 Ldistilled water, adjust pH to 6.5 with KOH. Then add: MgSO₄×7 H₂O 3 g,MnSO₄×H₂O 0.50 g, NaCl 1 g, FeSO₄×7 H₂O 0.10 g, CoSO₄×7 H₂O 0.18 g,CaCl₂×2 H₂O 0.10 g, ZnSO₄×7 H₂O 0.18 g, CuSO₄×5 H₂O 0.01 g,KAl(SO₄)₂×12H₂O 0.02 g, H₃BO₃ 0.01 g, Na₂MoO₄×2 H₂O 0.01 g, NiCl₂×6 H₂O0.03 g, Na₂SeO₃×5 H₂O 0.30 mg and Na₂WO₄×2 H₂O 0.40 mg, adjust pH to 7with KOH.

Medium basis: Add NH₄Cl 1 g, K₂HPO₄ 0.3 g, KH₂PO₄ 0.3 g, MgCl₂×6 H₂O 10g, CaCl₂×2 H₂O 0.1 g, KCl 1 g, sodium acetate 0.5 g, cysteine 0.5 g,trypticase 1 g, yeast extract 1 g, NaCl 100 g, trace element stocksolution 1 ml and resazurin 0.001 g to 1 L of distilled water.

Boil the medium basis, cool down under N₂:CO₂ (80:20 v/v). Aliquot underN₂:CO₂ (80:20 v/v) in culture tubes and autoclave. Add to sterile mediumbasis the following sterile stock solutions up to the concentrationsshown in parenthesis: 2% Na₂S×9H₂O (0.2 ml/10 ml), 10% NaHCO₃ (0.2 ml/10ml), 1M glucose (0.2 ml/10 ml) and 1M sodium thiosulphate (0.2 ml/10ml). Optionally adjust pH to 7. Like this, the growth medium isobtained.

Halanaerobium congolense (DSM 11287) is obtained from the GermanCollection of Microorganisms and Cell Cultures (DSMZ). Grow apre-culture at 42° C. under anaerobic conditions in the growth medium,thereby incubating for 7 days.

Provide 5 culture tubes (R0-R4), each with 2 ml of growth medium,wherein biostabiliser (in the form of hop extract, rosin in sodium saltsolution and myristic acid in sodium salt solution) is added to thegrowth medium in each culture tube up to the following concentrations:

Tube Hop Acid Resin Acid [ppm] [ppm] [ppm] Myristic Acid R0 0 0 0 R1 525 25 R2 20 100 100 R3 100 500 500 R4 200 1000 1000

R3 R4

Inoculate the tubes with 20 μl of pre-culture each and then determine,after 1, 2, 3 and 4 days of incubation at the growth conditionsmentioned above, the optical density (OD). A lower optical densitycompared to R0 is found, wherein the density difference to R0 increaseswith higher biostabiliser concentration. In addition, the amount ofrespectively produced H₂S can be determined.

Example 3

Biostabilising Effect on Halolactibacillus

Preparation of the Growth Medium:

Add peptone 5 g, yeast extract 5 g, glucose 10 g, KH₂PO₄ 1 g, MgSO₄×7H₂O 0.2 g, NaCl 40 g, Na₂CO₃ 10 g to 1 L of distilled water. Optionallyadjust pH to 9.6.

Halolactibacillus halophilus (DSM 17073) is obtained from the GermanCollection of Microorganisms and Cell Cultures (DSMZ). Grow apre-culture at 30° C. in the growth medium, thereby incubating for 3days.

Provide 5 culture tubes (R0-R4), each with 2 ml of growth medium,wherein biostabiliser (in the form of hop extract, rosin in sodium saltsolution and myristic acid in sodium salt solution) is added to thegrowth medium in each culture tube up to the following concentrations:

Tube Hop Acid Resin Acid [ppm] [ppm] [ppm] Myristic Acid R0 0 0 0 R1 525 25 R2 20 100 100 R3 100 500 500 R4 200 1000 1000

Inoculate the tubes with 20 μl of pre-culture each and then determine,after 1, 2, 3 and 4 days of incubation at the growth conditionsmentioned above, the optical density. A lower optical density comparedto R0 is found, wherein the density difference to R0 increases withhigher biostabiliser concentration.

Example 4

Biostabilising Effect on Halanaerobium and Halolactibacillus

The effect of selected biostabilisers (hop beta acids or resinacids/myristic acid, biostabiliser A or B) on the growth of threedefined bacterial strains (Halanaerobium congolense DSM 11287,Halolactibacillus halophilus DSM 17073, Halolactibacillus miurensis DSM17074) was analyzed by an in vitro experiment.

The following aqueous stock solutions for the selected biostabiliserswere used: (A) 10% alkaline beta hop acid solution (hop extract) and (B)20% alkaline solution of resin acids (rosin) and myristic acid (60:40).

TABLE 1 Culturing Conditions Environmental Strain Culture mediumconditions Halanaerobium congolense DSMZ Medium No. 933 3 days, DSM11287 (as in Example 2) anaerobic, 42° C. Halolactibacillus miurensisDSMZ Medium No. 785 48 h, DSM 17074 (as in Example 3) microaerophilic,30° C. Halolactibacillus DSMZ Medium No. 785 48 h, halophilus (as inExample 3) microaerophilic, DSM 17073 30° C.

Each of the three test strains was grown for several days before thebiostabilising experiments according to table 1. The species identitywas checked by sequencing and again by a sequence comparison in publicdata bases.

The biostabilising experiments were carried out with the Bioscreeninstrument. It involves a special microtiter plate photometer whichsimultaneously serves as an incubator and can accomodate up to twoso-called Honeycomb microtiter plates with 100 wells simultaneously. Thedetermination of the growth is carried out by an OD measurement at 600nm. During the incubation the Honeycomb microtiter plates werde shakenevery 15 sec before each measurement with medium strength for 5 sec. TheOD measurement was carried out every 15 min.

In each of the tests carried out two Honeycomb microtiter plates pertest strain were used which were each filled according to the samescheme. On the respectively first microtiter plate, the biostabiliser Awas tested and on the respectively second plate, the biostabiliser B wastested at concentrations of 0.5 ppm, 1 ppm, 10 ppm, 50 ppm, 100 ppm, and250 ppm. The concentration data in ppm in this example refer to thefinal concentration of hop acids in the growth medium (for A) and to thefinal concentration of resin acids/myristic acid in the growth medium(in the composition 60:40, for B). “ppm” in this example stands for mgof organic acids (i.e. hop acids or resin acids/myristic acid) per kg ofsolution (i.e. growth medium+additives).

All test strains were tested sevenfold (i.e. n=7) at each listedbiostabiliser concentration. To this purpose, the respectivebiostabiliser concentrations were investigated in parallel with eachbacterial strain in seven wells of the Honeycomb microtiter plate. Inaddition, three wells per biostabiliser concentration were included ascontrol means, i.e. instead of the bacterial suspension, sterile waterwas pipetted into the wells. In addition, seven wells were carried outwithout biostabiliser on each plate for further control to detect thetypical growth of each strain under the chosen test conditions.Sterility control included three additional wells each per biostabiliserand bacterial strain (medium without biostabiliser and without bacterialsuspension).

In each well, the respective growth medium according to table 1,bacterial suspension (or sterile water at the appropriate controls) andthe biostabiliser solution were pipetted at the appropriateconcentration. To create a strictly anaerobic atmosphere forHalanaerobium congolense, the growth medium was mixed with oxyrase(oxygen removing enzyme). By mixing all of the components, therespectively desired biostabiliser concentrations were achieved.Subsequently, all wells were overlaid with 2-3 drops of sterile paraffinoil. This served to maintain the anaerobic conditions for Halanaerobiumcongolense and to create microaerophilic conditions forHalolactibacillus miurensis and Halolactibacillus halophilus.

Composition of each volume in the wells of the microtiter plate (forHalanaerobium congolense)

300 μl 1.25×growth medium (DSMZ No. 933)

50 μl bacterial suspension

10 μl Oxyrase® (Oxyrase Inc., Ohio, USA)

10 μl biostabiliser solution at an appropriate concentration

2-3 drops of paraffin for overcoating

Composition of each volume in the wells of the microtiter plate (for theother three strains)

300 μl 1.25×growth medium (DSMZ No. 785 or CASO)

50 μl bacterial suspension

10 μl biostabiliser solution at an appropriate concentration

2-3 drops of paraffin for overcoating

The respective growth curves are shown in the figures and show a strongconcentration-dependent influence on the growth of the test strains bythe biostabilisers. At higher concentrations of the biostabilisers itcomes to an opacification of the growth medium (i.e. higher initial ODvalue—for an assessment of the biostabilising effect, it is not theinitial OD value which is relevant, but the course of the growth curveor the OD gain)—and occasionally to aberrations (because thebiostabiliser occasionally precipitates out of solution), yet thedose-dependent tendency towards biostabilisation is clearly evident fromthe figures.

In most tested biostabiliser/test strain combinations, a concentrationof 0.5 ppm is already causing an influence on the growth (lower OD gainor delayed reaching the maximum OD). A complete inhibition of growth(i.e. no OD enhancing growth occurs any more) appearedstrain-individually mostly at 10 ppm or 50 ppm of biostabiliserconcentration (see Table 2).

Under the test conditions, the biostabilisers A and B were able toinhibit the growth of the tested bacteria, i.e. to act biostabilising.

TABLE 2 Minimum biostabiliser concentration for total growth inhibition.A: hop acids, B: resin acids/myristic acid (60:40) Biostabiliser BStrain Biostabiliser A [ppm] [ppm] Halanaerobium congolense 10 100 DSM11287 Halolactibacillus miurensis 50 10 DSM 17074 Halolactibacillushalophilus 1 250 DSM 17073

Example 5 Comparative Example

The biostabilising effect of hop acids compared to the chemical biocidemethylenbis[5-methyloxazolidine] known in the art and used in a largetechnical scale, on Halolactibacillus. This test was operatedessentially in accordance with example 4 (except in respect of thebiostabilisers and the concentrations of them). The results of theseinvestigations are shown in FIGS. 4A-4C and 5A-5C. The dose-dependenttendency towards biostabilisation by hop acids is clearly evident.Furthermore it has surprisingly been shown during the investigationsthat the biostabilising effect of hop acids on Halolactibacillus isstronger even at lower concentrations than in the case of the chemicalbiocide methylenbis[5-methyloxazolidine] (see FIGS. 4B-4C and 5B-5C).

CITED NON-PATENT-LITERATURE

-   Ashraf et al. “Green biocides, a promising technology: current and    future applications to industry and, industrial processes.” Journal    of the Science of Food and Agriculture 94.3 (2014): 388-403-   Emerstorfer, Kneifel and Hein, “The role of plant-based    antimicrobials in food and feed production with special regard to    silage fermentation”, Die Bodenkultur—Journal for Land Management,    Food and Environment (2009), vol. 60, issue 3-   Jorgensen and Ferraro, “Antimicrobial susceptibility testing:    general principles and contemporaryPractices”, Clinical Infectious    Diseases 26, 973-980 (1998).-   Madigan et al. “Brock Biology of Microorganisms”, 10. Aus-gabe    (2003), insbesondere S. 138-   Ullmann's Encyclopedia of Industrial Chemistry, Vol. A 23 (1993), S.    73-88.-   Wang, Jifu, et al. “Robust antimicrobial compounds and polymers    derived from natural resin acids.” Chemical Communications 48.6    (2012): 916-918.-   White, et al. “Antimicrobial resistance: standardisation and    harmonisation of laboratory methodologies for the detection and    quantification of antimicrobial resistance” Rev. sci. tech. Off.    int. Epiz. 20 (3), 849-858 (2001).

1. A method comprising a use of a process fluid in a geothermalborehole, wherein the process fluid comprises a biostabiliser, whereinthe biostabiliser comprises at least one organic acid, or a salt,alcohol or aldehyde thereof, wherein the at least one organic acid isselected from the group consisting of hop acids, resin acids, fattyacids and mixtures thereof.
 2. The method according to claim 1, whereinthe process fluid is used as a drilling fluid in the geothermalborehole.
 3. The method according to claim 1, wherein the process fluidfurther comprises at least one defoamer.
 4. The method according toclaim 3, wherein the process fluid further comprises at least onegelling agent, wherein the gelling agent is a biopolymer or a polymericderivative thereof.
 5. The method according to claim 4, wherein theprocess fluid further comprises a water-softening agent.
 6. The methodaccording to claim 1, wherein the biostabiliser comprises: at least oneresin acid, or a salt, alcohol or aldehyde thereof, and at least onefatty acid, or a salt, alcohol or aldehyde thereof.
 7. The methodaccording to claim 1, wherein the biostabiliser is a mixture of at leasttwo of the following components: hop extract, natural resin and myristicacid or a salt thereof.
 8. The method according to claim 7, wherein thebiostabiliser is obtainable by adding at least two of the followingcomponents: hop extract, natural resin and myristic acid or a saltthereof.
 9. The method according to claim 1, wherein the biostabilisercomprises: a hop acid, selected from the group consisting of humulone,isohumulone, cohumulone, adhumulone, prehumulone, posthumulone,tetrahydroisohumulone, and tetrahydrodeoxyhumulone, lupulone,colupulone, adlupulone, prelupulone, postlupulone, hexahydrocolupulone,and hexahydrolupulone; or a resin acid selected from the groupconsisting of pimaric acid, neoabietic acid, abietic acid,dehydroabietic acid, levopimaric acid, and palustrinic acid; or a fattyacid selected from the group consisting of capric acid, undecylenicacid, lauric acid, myristic acid, palmitic acid, margaric acid, stearicacid, arachinic acid, behenic acid, lignoceric acid, cerotic acid,palmitoleinic acid, oleic acid, elaidic acid, vaccenic acid, icosenoicacid, cetoleic acid, erucic acid, nervonic acid, linoleic acid,linolenic acid, arachidonic acid, timnodonic acid, clupanodonic acid,and cervonic acid.
 10. The method according to claim 6, wherein: thetotal concentration of resin acids in the process fluid is 0.05-5000 ppmand the total concentration of fatty acids in the process fluid is0.05-5000 ppm.
 11. The method according to claim 1, wherein the processfluid further comprises at least one additional antimicrobial agentand/or biostabiliser.
 12. The method of claim 1, further comprisingbiostabilising a geothermal borehole by pumping the process fluid intothe geothermal borehole.
 13. A process fluid for a geothermal boreholecomprising: a biostabiliser comprising at least one organic acid, or asalt, alcohol or aldehyde thereof, wherein the at least one organic acidis selected from the group consisting of hop acids, resin acids, fattyacids and mixtures thereof; wherein the process fluid further comprisesclay minerals and a defoamer.
 14. The process fluid according to claim13, further comprising a gelling agent.
 15. A process for thepreparation of the process fluid according to claim 13, wherein theprocess fluid comprises water, comprising adding at least one organicacid or a salt, alcohol or aldehyde thereof to water or awater-containing portion of the process fluid, wherein the at least oneorganic acid is selected from the group consisting of hop acids, resinacids, fatty acids, and mixtures of two or all of them; the processfurther comprising adding clay minerals and a defoamer to obtain theprocess fluid.
 16. The method according to claim 1, wherein the processfluid further comprises at least one defoamer based on non-ionicsurfactants.
 17. The process fluid of claim 13, wherein the totalconcentration of resin acids in the process fluid is 0.05-5000 ppm andthe total concentration of fatty acids in the process fluid is 0.05-5000ppm.
 18. The method according to claim 1, wherein the process fluidfurther comprises at least one gelling agent, wherein the gelling agentis a polysaccharide of one of a starch, vegetable gum, xanthan,cellulose, polyanionic cellulose, or pectin.
 19. The method according toclaim 1, wherein the process fluid further comprises at least one ofacetic acid, lactic acid, propionic acid, benzoic acid, sorbic acid,formic acid, and salts.